1. Field of the Invention
The present invention relates generally to DSL reference noise cancellers.
2. Background Art
Digital subscriber line (DSL) technologies provide potentially large bandwidth for digital communication over existing telephone subscriber lines. Asymmetric DSL (ADSL) and very-high-speed DSL (VDSL) can adapt to the characteristics of the subscriber line by using a discrete multitone (DMT) line code that assigns a number of bits to each tone (or sub-carrier). The number of bits can be adjusted to channel conditions as determined during initialization and subsequent on-line reconfiguration known as “bit-swapping” of the DSL modems at each end of the subscriber line.
ADSL and ADSL2+ services operate in the frequency range of 138 KHz to 2.2 MHz. The nearly 5,000 AM radio stations in use in the United States operate at frequencies in the range of 540 KHz to 1.7 MHz. These radio signals are broadcast generally and can interfere with DSL modem operations. In addition, other sources of radio frequency (RF) interference, including, but not limited to ham radio signals, crosstalk, impulse noise and other man-made electronic radiation, can contribute to a deterioration in DSL system performance as a result of the interference they cause.
Very-high-bitrate DSL (VDSL or VHDSL) provides faster data transmission than ADSL or ADSL2+ over a single flat untwisted or twisted pair of copper wires (up to 52 Mbit/s downstream and 16 Mbit/s upstream), and on coaxial cable (up to 85 Mbit/s down- and upstream). VDSL utilizes the frequency band from 25 kHz to 12 MHz. These fast speeds mean that VDSL is capable of supporting high bandwidth applications such as HDTV, as well as telephone services (voice over IP) and general Internet access, over a single connection. VDSL is deployed over existing wiring used for POTS and lower-speed DSL connections.
Second-generation VDSL (VDSL2) utilizes frequencies of up to 30 MHz to provide data rates exceeding 100 Mbit/s simultaneously in both the upstream and downstream directions. The maximum available bit rate is achieved at a range of about 300 meters; performance degrades as the loop attenuation increases.
Currently, standard VDSL uses up to 7 different frequency bands. This enables data rates to be customized between upstream and downstream depending on the service offering and spectrum regulations. First generation VDSL standard specified both quadrature amplitude modulation (QAM) and discrete multi-tone modulation (DMT).
Throughout this disclosure, ADSL, ADSL2+, VDSL, and VDSL2 will be collectively referred to as xDSL.
On transmission lines in DMT communication systems, such as xDSL, the data signal is generally transmitted differentially. Interference such as radio-frequency interference (RFI), crosstalk and impulse noise electromagnetically couples into both the common mode and the differential mode of such transmission lines. In the case of a binder containing multiple transmission lines, such interference may couple into some or all of the transmission lines in the binder and such noise may be correlated between lines.
Conventional techniques for reducing differential noise, thereby improving data rates over the xDSL, include use of common-mode information. In a traditional xDSL system, the common-mode voltage is measured relative to chassis or board ground, an estimate of the differential-mode interference is constructed and the interference estimate is subtracted from the desired signal.
RF interference often couples most strongly to telephone lines between customer premises equipment (CPE) and pedestals (i.e., service terminals) and the like. Pedestals offer a cross-connection point between lines going from a central office (or remote terminal central office) to a specific customer premises or a few customer premises (often referred to as a “drop”). The remainder of lines from the Central Office may continue to other pedestals. Typically, there are 2-6 lines in the “drop” segment to each customer. The relatively exposed DSL transmission loop segment running between the pedestal and customer premises acts as an antenna, picking up the RF interference signals, especially the AM and ham radio broadcasts in the area, and even interference from appliances in the home. This segment is often not well shielded or employs shields that are not well grounded, leading to additional receipt of RF signals by the telephone line(s).
It is well known that the common-mode voltage of an xDSL loop can be used as a reference signal for noise and interference cancellation. Traditionally, the common-mode voltage of the xDSL loop is taken relative to board chassis ground for use in a digital noise canceller. However, board or chassis ground can have its own noise sources, which can interfere with the measurement of the common mode noise from the xDSL loop and thereby reduce performance of the noise cancellation system. This introduces a level of complexity and additional components that it would be desirable to eliminate.
Service providers often use loop length to estimate the data rate that they can offer to the customer. Clean loops without much noise and interference may not benefit much from noise cancellation and may already have a high potential data rate. Loops with a high level of interference will generally benefit most from noise cancellation. With noise cancellation, a set of loops of the same length will have more similar data rate potentials, allowing the service provider to guarantee a higher minimum data rate based on loop length alone. For example, a low-noise 12,000 foot ADSL loop may have a potential data rate of 5.0 Mbps but a high-noise 12,000 foot ADSL loop with a high level of AM radio interference may have a potential data rate of only 2.0 Mbps without noise cancellation.
What is needed is a noise cancellation system that will improve data rates of high noise xDSL loops so that data rates can approach maximum potential for a given loop length.